1. FIELD OF THE INVENTION
This invention relates to the art of correlation spectrometry and more particularly to an improved correlation spectrometer which can detect and quantify a number of specific gases or vapors in a mixture of gases or vapors with relative ease and speed as compared with prior art correlation spectrometers.
In protecting the earth's fragile environment while at the same time preserving an efficient industry necessary for an industrial society, a need has arisen to simply and economically check the waste products of our industrial processes for certain noxious or undesirable gases and vapors. It is possible, in the absence of gases or vapors with interfering spectra, to set a spectrometer to a band of wave lengths corresponding to one peak of absorption of the pure gas component of interest (COI) and, with suitable calibrations, to quantitively determine the total amount of the COI in the optical path. In the presence of interfering gases, the location of the band must be chosen so that absorption in that spectral region is unique to the COI; however, this is often difficult to achieve because most gases have rather complex absorption spectra. Particularly, omnipresent components, such as water vapor, have absorption spectra which mask many COI's.
In addition, a single line system has low sensitivity and poor specificity. Both qualities can be improved by using more of the absorption spectrum than a single line.
2. DESCRIPTION OF THE PRIOR ART
In the prior art, an effective and a sensitive apparatus for detecting and measuring the concentration of a pure gas COI in a mixture of gases and/or vapors is to correlate a spatially dispersed radiation absorption spectrum of the mixture or sample thereof with an optical mask representing the absorption spectrum of the pure gas COI.
Prior art correlation spectrometers commonly use a transmission mask which is a replica of the COI in the sense that those regions of the mask which correspond to absorption maximum are left opaque, and those which correspond to absorption minimum are transparent. A detector, such as a photomultiplier tube, is exposed to greatly increased radiation when the mask and the absorption spectrum are aligned. The mask is periodically shifted relative to the spectrum so that the radiation incident on the detector falls to a lower level as the mask and the spectrum pass from correlation to anti-correlation.
Reference may be had to the following materials for a more detailed description of correlation spectrometers and other related works:
A. R. Barringer, "Chemical Analysis by Remote Sensing", presented as a High Session at the 23rd Annual ISA Instrumentation Automation Conference, October, 1968;
A. R. Barringer and J. D. McNeill, "Advances in Correlation Techniques Applied to Spectrometry", presented at the National AID Symposium of the ISA, New Orleans, May, 1969;
A. Bessen, "Pattern Recognition Logic Analyzes Infrared Signals", Electronics, Nov. 11, 1968, pp. 112-118;
U.S. Pat. No. 3,820,901, "Measurement of Concentrations of Components of a Gaseous Mixture", June 28, 1974;
U.S. Pat. No. 3,807,876, "Gas Densitometer", Apr. 30, 1974;
U.S. Pat. No. 3,790,798, "Method and System for the Infrared Analysis of Gases", Feb. 5, 1974;
U.S. Pat. No. 3,447,876, "Apparatus for Detecting Monatomic Vapours", June 3, 1969;
U.S. Pat. No. 3,449,565, "Apparatus for Absorption Analysis Using a Source Having a Broadened Emission Line", June 10, 1969;
U.S. Pat. No. 3,549,260, "Spatially Dispersive Correlation Interferometer", December 22, 1970;
U.S. Pat. No. 3,518,002, "Spectrometer", June 30, 1974; and
U.S. Pat. No. 3,578,980, "Spectral Analysis Using Masks Having Different Combinations of Transmitting and Non-Transmitting Portions", May 19, 1971.
Correlation spectrometers, while generally satisfactory, are limited in that only one COI can be monitored without changing the mask; a time consuming and difficult task, particularly if the spectrometer is located in a remote location. Two COI's can be monitored by using parallel paths, but this apparatus is cumbersome and expensive.
An object of this invention is the provision of a correlation spectrometer which can perform multi-gas analysis very rapidly without the problem of mechanically exchanging masks and realignment of the instrument. An additional object of the invention is the provision of an instrument which can perform sophisticated correlations and thus provide increased detection sensitivity.